Cheap PA with fidelity

(ancient) Guaytone SX1Problem

This little guitar amp, that had been used for a PA system with a dynamic microphone, started picking up the radio station at one location it was used, and has a significant amount of background noise. It might be fixable, but probably not by me.

What to do?

In looking for new portable PA systems, I didn't find anything that appeared like it would produce quality sound, was adequately portable, and also cheap. This Audio Choice C100 system looks nice, but is rather large and heavy, and I couldn't find the manufacturer's web site, which makes me wonder if it is still in production. This Peavey Messenger looks nice, has all the desired features, is nicely portable (although a little heavy at 20+ pounds) but at $500 was a bit expensive for my blood. Earlier models of the Roving Rostrum have demonstrated themselves to be rugged, reliable, easy to set up, with long battery life, and mediocre but adequate sound quality. Perhaps these newer models have better sound quality, and they did up the power rating from 35 to 50 Watts, but for $700, even the Peavey looked like a better deal. This portable Karaoke machine seems to come in at least a couple models with different power ratings, 10 Watt, and maybe 30 Watt, but it looks like cheap plastic junk. And it is hard to find technical specs on the audio quality for some of these units, too. Fender has some nice looking units, but early models had poor quality speakers, and the larger units are quite heavy to carry around. Newer models claim to have better speakers, and hopefully they do. Prices from $400 and up for the larger units put me off, and while the smaller systems are really cute, they only have one speaker, although that makes things simpler and the system being replaced also only has one speaker. Pricing for the P10 is about $200, so it might be competitive. Given that the bigger units initially had poor quality speakers, though, it made me a little shy about their smaller unit with only one speaker.


About that time, I read a review in PC Magazine for Portable Speakers with Power. While some of the units sounded interesting, many seemed to have drawbacks for use as a portable PA system component. None of the units have a microphone input of any sort... some have only an iPod docking port, although most have a "line level" input also. In the review of their favorite unit, the Altec Lansing, they referred to an earlier review of another item which seemed quite interesting, the 40 Watt Virgin Electronics Boomtube EX, to which they had also given an Editors Choice award. While they considered it pricey, the prices have apparently come down since that review back in Feb 2005, as Amazon has it for much less than list price.

The problem with the Boomtube and competitive devices for PA system use is their lack of microphone inputs. So I went shopping for a portable preamp. Many such exist, but most of them are either in the several hundred dollar range, or else seem to aimed at using micro stereo microphones. After extensive searching, I discovered the Applied Research and Technology MicroMIX, which is much cheaper at B&H and elsewhere. While out of the box it requires AC power, the wall wart converts that to 12V DC 150 mA, so a small battery pack could probably be made to work with it. You'd need to find a power connector with 2.5 mm inner diameter, and 5 mm outer diameter to hook it up.

What I did

I had a preamp/mixer and some microphones on hand for testing, so I decided to obtain the Boomtube EX and see how it sounded. Initial testing at home was positive, so I took it to a small gathering, and did a "head-to-head" comparison with an old Roving Rostrum. Everyone unanimously voted as preferring the Boomtube. So then I went ahead and ordered the ART MicroMIX. It also sounded good in home testing, and I used it the next week at a similar gathering, again with positive results. all the parts and wiresSomeone there noticed that my 1/4" to 1/8" adapter wasn't stereo, and that all my positive results had been gathered with only one active channel. Now when I'd first found and used that adapter during initial setup, the thought crossed my mind that it might not be sufficient for converting mono to stereo input, but I wanted to hear the quality, and only had a mono signal anyway, and then I kind of forgot about it. But now I had another problem... too many wires, and too many parts... see the boomtube in the background, the MicroMIX is almost obscured by the mike wire, and there are two wall warts plugged into the extension cord.

So I figured, hey, let's jam all this stuff into a project box.... just check Radio Shack to find the right size... I figured I needed something 10-12" long, 4-6" wide, and 2.5-4" high, so that I could cram the MicroMIX, both wall warts and the end of an extension cord into the box, including rolls of wire from the wall warts to the MicroMIX and the boomtube, and still have room to mount a microphone jack..... oops, they have nothing that big. Not even close! Back to a broader internet search, and I found nothing. I remembered a friend had made a nice laptop carrying case out of some sort of plastic, and I thought I'd see him again soon and would ask again for the details of how he did it. On the way to the place where I figured I'd see him, I passed the Fry's on Hamilton Avenue in Campbell, CA. With a little time to spare in my schedule, I ran in to see if they had project boxes... and indeed they did, on aisle 8B. project boxAnd they had a much greater variety than Radio Shack! Including one that fit my dimensions at the small end: 10x4x2.5". I'd sort of hoped to find something with at least one dimension bigger than my minimums to be sure everything would fit, but it was that or nothing, so I bought it. It is an LBM #144 by LMB Heeger Inc, 6446 Flotilla St, Commerce CA. They have a variety of sizes, but their biggest of that style seems to be the one I found. The do have bigger ones in their Flangelock style of chassis boxes.

So now I had to actually do some skilled assembly work, to avoid which is why I've done software for my whole career in spite of having a EE degree... Actually, the EE part wasn't too bad... I do still have a soldering iron that a former neighbor gave to me, was able to locate the solder, and the clamp jig for holding the parts! The two electrical parts to be fabricated were both wires. The source for both wires was a right-angle male to right-angle male short "jumper" wire that I had on hand. That way, one end of each wire was already done, and the right-angle connectors allowed the project box to be only 4" wide. Straight connectors would have required another inch or two of width in the project box.

One was an extension wire for the microphone input, so that the microphone jack could be on the same side of the project box as the volume controls (the MicroMix has them on opposite sides, but it is smaller, so it has to). Radio Shack did have the mountable 1/4" phono jack (but I had to buy a pair), and some grommets to protect wires passing through holes in the box (I had to buy a whole assortment pack, but they were cheap). I planned to put the new microphone jack to the right of the MicroMIX in the project box, so that its open wires would be mostly surrounded by metal of the MicroMIX or the project box walls. Although I did also put electrical tape on those nearby metal surfaces to avoid shorting things out. That is just because I really, really don't want to open up the box again once it is made; there was adequate clearance that no shorts should occur even without the tape. The second wire was for the audio output. As shown below right, this had to include a "conversion" from mono to stereo, so you see that the center wire is soldereded to both the left and right channel connectors.

 soldering iron, wires stereo adapter

Now for the box modifications. While it would have been nice to have had a drill press, I'd have to do a lot more work to justify having one, and I don't plan to do that much. So here were the major tools:

 drill bits bench vise Dremel tool and bits

I'd strongly recommend that anyone that might try to duplicate my efforts would do two things I didn't. Not doing them really doesn't save time. First, there are 4 items on the front of the MicroMIX: power indicator LED (green), Phantom Power pushbutton switch, Phantom Power indicator LED (red), and the input GAIN control (rotating knob). At first I thought I'd cut out a big section so you could see the face, but the legends were on the top of the MicroMIX box so there was little benefit to reducing the strength of the box that much... 4 drilled holes would expose everything that needed to be exposed. And then a 5th hole for the 1/4" in mike jack to the right. So what I should have done was make a paper template to position those four items, placed it on the face of the MicroMIX, and measured it to the top of the wall of the box. And then transferred it to the outside of the box, and drilled through the template. And the second thing is that I should have found a block of wood to sit inside the box, so that when the drill bits got through the metal, they'd have the wood to sink their teeth into, instead of bouncing around and making the holes more triangular than round... the jaws on the vise weren't big enough to hold the top of the box in the middle, because the bottom of the box got in the way, so I could only clamp the end, and drill several inches away. Having a chunk of wood in the box would have made the clamping easier too. Did I mention it would have been nice to have a drill press? Anyway, I got some holes drilled, and the little diamond grinder you see in the Dremel tool above was a lifesaver in cleaning the edges of the holes, and making them round again, and adjusting the hole for the GAIN knob until it would actually fit....
 project box bottem, with parts stuffed in

Oh, and that other bit in the Dremel picture came in real handy for the slot on the left edge of the box for the power wire. Because both ends of the extension cord were molded, large, and I didn't want to cut it and mount power plugs on the project box, both for space and time reasons, and I didn't want to cut a hole big enough for the end of the extension cord to pass through, a slot at the edge, complete with grommet, seemed the most appropriate solution. The other bit is a "rotary saw" bit, which I had bought when I happened to see them available, because it looked like a "neat accessory" for the Dremel. Sometimes those impulse buys pay off! It was the perfect bit for the job. There was also a similar need for a slot in the top of the box, through which could pass both the 12V pulsed DC output of the boomtube's wall wart, and the output signal from the MicroMIX to the boomtube.

Note the two coils of wire in the picture above, inside the box (and tied with wire twisties). To avoid making an inductor out of coils of wire, and picking up all sorts of in appropriate RFI noise from the atmosphere, it is good to place both ends of the coil together, and then coil both wires of the resulting loop together. That way the signal picked up in one winding is counteracted by the signal in the other winding, which is going the opposite direction (with respect to the flow of the main signal through the wire). This is similar to the theory of how balanced audio signals work. Now being in an aluminum box helps avoid RFI noise too, but there is another reason to wind the coil in this manner... to avoid generating magnetic fields of our own, which could get picked up at the other end of the box by the unbalanced microphone wire. Oh, and if you ever want to uncoil the wire again, it turns out that you can avoid tangles by starting at the loop, and shaking the rest of the wire free. Keep that in mind when winding up loose external cables like microphone or speaker wires.

The final modifications to the box were four slots in the top so that the boomtube can be strapped to the project box. The boomtube didn't come with any mounting brackets, so straps seemed more appropriate than drilling holes through the case and whatever important stuff happened to be inside right there... and I didn't want to take it apart, after all it is brand new and still under warranty! So a couple industrial strength plastic ZIP ties, having abouth the same width as some of the trim pieces on the boomtube, don't look too bad, and I hid the ends of the ZIP ties inside the project box. Here is what the audience sees:
 assembled PA, audience view

All smooth aluminum, the rectangular base, and the cylindrical top, plus the two detached tweeters.

And the speaker/operator sees basically the same thing, but with all the wires and controls.
 speaker/operator view

Here's another shot, showing all the controls a little closer, and the connections to the boomtube.

 connections and controls

Notice how the power wire (left) and audio input (right) wires to the boomtube come from the grommeted slot in the top of the box. The two middle wires are standard RCA plug speaker wires. The ones that came with the boomtube are only 3' long, but longer ones can be obtained if desired. I did stick labels on all the controls and connections after I took this picture. The microphone is plugged in, in these pictures. It is one I bought from DAK for $20 nearly 20 years ago, and they are great little electret microphones, running on one AA battery. I'd be glad to learn about any equivalent, inexpensive imcrophone available on the market today (here's a scan of the microphone specs).

packedSosuitcase in the end, a bunch of messy stuff got hidden, and there are 2 speaker wires, one power wire, and one microphone and wire, plus the PA package itself. It can be neatly packed into an ancient soft-side briefcase someone threw away at our house some years ago.

My out-of-pocket costs were $141, and I used a few items I had on hand
The total cost, then, is probably about $180, about the cost of some of the lower end PA systems. In putting this page together, I notice that Amazon has reduced the price of the boomtube EX by $10 since I bought this one! It is a neat little unit, with good sound quality. You could use it as laptop speakers for your computer, for your portable CD or MP3 player, or you could do what I did, and make a portable PA system that has better audio quality than most price competitive units. Maybe I'll get another unit for use with the laptop.

Alternate solution

After I was mostly done with the above project, someone pointed out the Roland Micro-CUBE. This item would be nearly a direct replacement for the original Guaytone, but is fairly low power (the Guayatone doesn't have a rating plate, and is too old to be found on-line, so it is hard to compare). For use as a PA system, one would need to keep the effects turned off! The person that brought this device to my attention thought that it might prove adequate for groups of 50 people in a room with average acoustics. It is cheap, light, battery operated, easy to set up, and appears rugged. Reviews on the internet are mostly positive, although many speak of it being low-power (2 Watts).   P.S. The someone that pointed out the Roland unit is now a happy Boomtube EX user!  Know anyone that wants a Roland?  LOL